Space vehicle system for determining earth&#39;s ultraviolet radiation limb

ABSTRACT

An optical system is used to scan the earth&#39;&#39;s horizon and project an image from which position information is derived for use in a space vehicle navigation control system. The image is projected onto a camera tube target through ultraviolet filters. The system thus utilizes the earth&#39;&#39;s ultraviolet radiation limb as an earth-space boundary reference. A radioactive reference graticule is positioned in contiguous relationship with the camera tube target to provide both target intensity calibration and a spatial check of the optical image.

250-203. K A 2% M Unlled I3IS Patent 1 1111 3,715,594

Drohan et al. 1 1 Feb. 6, 1973 1 1 SPACE VEHICLE SYSTEM FOR 3,201,5918/1965 Froelich et al ..250/2o3 x DETERMINING EARTH '5 3,230,376 1/1966Goetze et al. ..2so/203 ULTR VIOL T RADIATION 3,519,823 7/1970 Heller etal. ..250/203 X 3,524,710 8/1970 Rickert ..356/25l [75] inventors:William A. Drohan, Bedford; Juri valge helmjsfordi David PrimaryExaminer-Benjamin A. Borchelt Warner Cambridge of Mass AssistantExaminer-N. Moskowitz [73] Assignee: The United States of America asAttorney-Harry A. Herbert, Jr. and Willard R.

represented by the Secretary of the Matthews, Jr. United States AirForce 22 Filed: Aug. 27, 1969 [57] ABSTRACT [21] APPL No; 853,549 Anoptical system is used to scan the earth's horizon and project an imagefrom which position information is derived for use in a space vehiclenavigation control [52] US. Cl. ..250/83.3 UV, 250/7], 250/203, systemThe image is projected onto a camera tube 356/251 target throughultraviolet filters. The system thus util- [Sl] Int. Cl ..G01t 1/16 izesthe earth's ultraviolet radiation limb as an can [58] Se'rch-250/71'833uvizmisswzsl space boundary reference. A radioactive reference graticuleis positioned in contiguous relationship with [56] References Cited thecamera tube target to provide both target intensity UNITED STATESPATENTS calibration and a spatial check of the optical image.

3,175,089 3/1965 Tulley et al ..250/203 1 Claim, 2 Drawing Figures 240/W0 21' I I! NC 6' 62477604! and,

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,qup- 75671! a dawn/Mar! Gina-Rum 34 as Mn/TM arr/cs #1422159 lili 3Jcmwws 6 a 7 MIC'I/INIJM SPACE VEHICLE SYSTEM FOR DETERMINING EARTH'SULTRAVIOLET RADIATION LIMB BACKGROUND OF THE INVENTION This inventionrelates to space vehicle navigation systems and in particular to meansfor determining the spacecraft's relationship to the center of the earthfrom earth-space boundary information When a space vehicle is in suchclose proximity to l the earth that it cannot be considered a pointsource for navigational purposes, then positional information can beobtained by scanning the earths horizon and establishing at least threereference points. A coordinate computer can from such informationdetermine the earths center and the space vehicle's relationship to it.Systems of this type are well known. US. Pat. No. 3,246,160,Photosensitive Horizon Scanner For Space Vehicle, issued Apr. 12, 1966to J. S. Zuckerbraun, discloses such a system in detail. The earths limb(or horizon, as it appears visually) does not provide information thatis precise enough for all navigational purposes, however. This is due tocloud cover, light wave aberrations, earth albedo, and the scattering ofsolar energy in the earth's atmosphere. Attempts have therefore beenmade to develop navigational systems based upon the detection of somephenomenon having a known directional aspect with respect to earth. Inthis regard the infrared limb (or detected spherical shell coincidingwith the earth's horizon at infrared frequencies) has been used in thepast. Infrared radiation is thermal radiation originating in the thermalenergy of the earth itself and its surrounding atmosphere. It isessentially independent of the relative position of the sun due to thediurnal stability of thermal processes in the atmosphere. It has been anattractive reference for satellite attitude control systems because ofthe continued need for local vertical information. The infrared limb,however, is affected by the temperature of the earths surface and thevarious atmospheric layers, and by the absorption of water vapor andcarbon dioxide in the atmosphere. Furthermore, the wavelength gradientover the infrared band is not sharp enough to provide the very narrowband of descrete detectable radiation required for an accuratenavigational reference.

The above review of the state of the art indicates that there currentlyexists a need in the field of satellite navigation for a planet-spaceboundary reference that can be more sharply defined than visual andinfrared planet limbs. There of course also exists a need for apparatuscapable of detecting such a reference. The present invention is directedtoward solving these and other problems.

SUMMARY OF THE INVENTION It is a basic concept of the invention toutilize the earth's ultraviolet limb as an earth-space boundaryreference. By means of theoretical study and direct measurements it hasbeen determined that an ultraviolet radiation detector situated outsidethe earth's atmosphere and operating within a 2,400 A to 2,700 A rangewill see only a very narrow ring of light at an apparent altitude ofapproximately 61 km. The ultraviolet air glow in this spectral range isconsiderably less intense than the limb and the radiance peak is uniquein that there exists no secondary peaks and no spurious meteorologicalfeatures. Furthermore, the altitude of the peak remains essentiallyconstant over a wide range of scattering angles and is verticallyunaffected by meteorological variables and changes in tropospheric andstratospheric compositions.

The present invention comprises a novel system adapted to detect theearth's ultraviolet limb and extract therefrom satellite positionalinformation for use in the satellite navigation system. In essence thesystem 0 comprehends the combination of an image forming opticalsubsystem, ultraviolet filters and a camera tube of the vidicon type.The optical subsystem includes a scanning mechanism that permitsscanning of the earth's horizon. The image formed by the opticalsubsystem is projected through the ultraviolet filters onto the targetof the camera tube. The camera tube scan circuit generates signals whenthe ultraviolet limb image appearing on the target is crossed by thetubes scanning beam and such signals are delivered to a coordinatecomputer for processing. A spatial check of the optical image as well asa reference for absolute sensitivity of the camera tube target is alsoprovided in the system by the inclusion of a novel radioactive referencegraticule located in the vicinity of the camera tube target.

It is a principal object of the invention to provide a satellitenavigation system that utilizes the ultraviolet limb as an earth-spaceboundary reference.

It is another object of the invention to provide a satellite navigationsystem that is simpler, more reliable and more accurate than existingsystems.

It is another object of the invention to provide a system for detectingthe earth's ultraviolet radiation limb and extracting satellite positioninformation therefrom.

It is another object of the invention to provide in a system fordetecting the earths ultraviolet radiation limb, a radioactive referencegraticule adapted to provide a spatial check for optical images and asensitivity reference for the system camera tube target.

These, together with other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription when taken in conjunction with the illustrative embodimentsin the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the preferredembodiment of the invention; and

FIG. 2 is a plan view of the radioactive reference graticule of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The major component parts of theinvention are illustrated in block diagram form in FIG. I. Although theconcept of an ultraviolet earth limb detecting system and the particularcombination or arrangement of component parts of the system hereindisclosed are novel, the individual elements and circuits that comprisethe system are known and commercially available. Consequently,considerable latitude is possible in the actual engineering design ofsuch a system. The energy gathering optics system 3, for instance, canbe of any appropriate optical design. For instance, it may be a narrowangle system with maximum scanning angle effected by scanning mechanism4 or it may be a wide angle optics system with no scanning whatever. Thechoice of ultraviolet band pass filter 5 may also be dictated byparticular design constraints and applications. Operation of the systemin the ultraviolet band between 2,400 A and 2,700 A has been determinedto be particularly effective. However, filters defining any other rangewithin the ultraviolet band are also with the scope of the invention andmay be preferable for some applications.

In operation radiant energy enters the energy gathering optics system 3,passes through filter 5 and is projected on the photosensitive target 8of detector 7. Detector 7 scans its target 8 with a line nominally inthe direction perpendicular to the horizon. This is also the maximumgradient for spatial distribution of radiant energy. The output signalof detector 7 is then a record of energy versus time. As long as thescanning of the phenomenon is performed linearly with time, a record isalso obtained of energy observed as a function of scanned angle. Thisinformation can be stored on a storage tube and subsequently transmittedto a telemetry station or it can be delivered to the satellitenavigation system coordinate computer for processing.

Spatial and intensity calibration for the system is provided byreference graticule 6. The graticule consists of a thin suprasil quartzplate 9 having a discrete pattern 20 etched onto one surface asillustrated in FIG. 2. The etched surface is coated with a longhalf-like radioactive material mixed with phosphor. The radioactivematerial emits low energy particles that are sufficient to activate thephosphor. The pattern 10 can be of any arbitrary configuration andconveniently is such that large areas occur at filter junctions andaround edges not used. Graticule 6 is positioned in the immediateproximity of target 8 and emits radiant energy to the target. If a scanof an entire frame over the target place is made, the pattern givesinformation of absolute sensitivity of the target over its entire arearegardless of amplifier power supply change. It also provides a checkspatially since the optical system is fixed and so is the pattern.Therefore, if the electronic scan amplitude or linearity varies, theoutput signal containing information from the pattern shows the presenceof variation, and quantitatively measures it.

While the invention has been described in its preferred embodiment it isunderstood that the words which have been used are words of descriptionrather than words of limitation and that changes within the purview ofthe appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

What is claimed is:

1. In combination with a satellite navigation system, means fordetecting the earth's ultraviolet radiation limb and extractingsatellite position information therefrom comprising:

image forming optical means,

means for scanning said image forming optical means,

ultraviolet bandpass filter means adapted to pass radiant energy havingwavelengths of not less than 2,400 A and not more than 2,700 A, detectormeans having a target responsive to radiant energy, said 0 tical meansand said filter means being arrange to provide ultraviolet filtering ofimages formed by said optical means and to display said filtered imageson said detector target, and

reference graticule in contiguous relationship to said detector target,said reference graticule consisting ofa quartz plate having a discretepattern of long half-life radioactive material mixed with phosphordisposed thereon.

i i i

1. In combination with a satellite navigation system, means fordetecting the earth''s ultraviolet radiation limb and extractingsatellite position information therefrom comprising: image formingoptical means, means for scanning said image forming optical means,ultraviolet bandpass filter means adapted to pass radiant energy havingwavelengths of not less than 2,400 A and not more than 2,700 A, detectormeans having a target responsive to radiant energy, said optical meansand said filter means being arranged to provide ultraviolet filtering ofimages formed by said optical means and to display said filtered imageson said detector target, and a reference graticule in contiguousrelationship to said detector target, said reference graticuleconsisting of a quartz plate having a discrete pattern of long half-liferadioactive material mixed with phosphor disposed thereon.